Molecular weight dependent water uptake and dynamics in lignin-based epoxy anticorrosive coatings

While the challenges of incorporating raw softwood kraft lignin (KL) as particulate pigments in epoxy coatings were addressed using mechanically sieved, size-fractionated lignin, and its corrosion resistance was evaluated, a key question remains: does solvent fractionation, which yields lower molecular weight lignin, offer improved barrier performance? This study explores the use of lignin as a functional particulate pigment in an amine-cured epoxy-based anticorrosive coating (EP), with a particular focus on how its molecular weight influences water uptake and transport behavior. Three pigmented epoxy systems with well-defined particle size distributions were investigated: a high molecular weight (M_w) kraft lignin incorporated via mechanical sieving (KL-sieved-EP, 5–30 μm), a low M_w lignin fraction obtained through ethyl acetate solvent fractionation (KL-EtOAc-EP, 5–10 μm), and a conventional acicular-shaped iron oxide pigment (FeOOH-EP, ∼0.3 × 0.5 μm). To evaluate the barrier performance of these coatings, water uptake behavior was systematically analyzed using gravimetric mass gain and electrochemical impedance spectroscopy (EIS), enabling assessment of both surface and bulk water transport pathways. The results show that KL-sieved-EP had the highest equilibrium water uptake, compared to KL-EtOAc-EP, which exhibited the lowest water uptake and diffusion coefficients, indicating superior compatibility with the epoxy matrix and a more cohesive barrier. These differences can be linked to the extent of supramolecular aggregation in the lignin: high molecular weight lignin tends to retain strong supramolecular associations, which limit its interaction with the epoxy. In contrast, solvent-fractionated low molecular weight lignin disrupts this aggregation, allowing for enhanced dispersion and integration within the epoxy network. The conventional FeOOH-EP coating showed a similar equilibrium water uptake to KL-EtOAc-EP based on gravimetric analysis but exhibited the highest diffusion coefficient among the three systems, indicating faster water transport. Although both had comparable total water uptake, EIS measurements revealed higher overall water absorption in FeOOH-EP. In contrast, KL-EtOAc-EP maintained extremely low water uptake in both gravimetric and EIS measurements. This contrast highlights a key finding: lignin-based systems, particularly KL-EtOAc-EP, showed no clear pigment–binder interface under SEM, which may contribute to their more uniform, defect-free morphology and improved barrier performance.